In general, it is desirable to obtain good performance and high capacity in wireless communication networks, and there are always on-going developments for improving and optimizing the network operation. However, many of these developments provide improved performance or useful functionality in some aspect, but may for example require additional resources, thereby leading to reduced performance in some other aspect.
For example, in legacy-based cellular communication networks such as networks operating according to the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, reference signals are normally transmitted or broadcasted in an always-on manner, e.g. to allow wireless communication devices served by the network to perform measurements on neighbor cell candidates and send measurement reports to the network side. The reference signals are easy to measure and yield consistent results, but the static always-on signaling leads to a high network resource utilization, interference and energy consumption. The measurement reports may be sent by the wireless devices only when some specific criteria are fulfilled to avoid too many unnecessary reports. However, there is still a demand for improved solutions for managing reference signals and related measurements and reports.
In the 5th generation (5G) of mobile communications, although not yet fully defined, wireless access will be realized by the evolution of Long Term Evolution, LTE, for existing spectrum in combination with new radio access technologies that primarily target new spectrum. Thus it includes work on a so-called 5G New Radio (NR) Access Technology, also known as 5G. The NR air interface targets spectrum in the range from sub-1 GHz up to 100 GHz with initial deployments expected in frequency bands not utilized by LTE.
Due to the scarcity of available spectrum in the range of frequencies that have so far been used for wireless communication, spectrum located in very high frequency ranges, such as 10 GHz and above, are planned to be utilized for future mobile communication systems.
For such high frequency spectrum, the atmospheric penetration and diffraction attenuation properties can be much worse than for lower frequency spectrum. In addition, the receiver antenna aperture, as a metric describing the effective receiver antenna area that collects the electromagnetic energy from an incoming electromagnetic wave, is frequency dependent, i.e., the link budget would be worse for the same link distance even in a free space scenario, if omnidirectional receive and transmit antennas are used. This motivates the usage of beamforming to compensate for the loss of link budget in high frequency spectrum.
Hence, future communications networks are expected to use advanced antenna systems to a large extent. With such antennas, signals may be transmitted in narrow transmission beams to increase signal strength in some directions and/or to reduce interference in other directions. The beamforming will enable high data rate transmission coverage also to very distant users which would not realistically be covered with normal sector-wide beams, which have lower antenna gain. Beamforming may be used at the transmitter, at the receiver, or both. In a large part of the spectrum planned for 5G deployments, the preferred configuration is to use a large antenna array at the access node and a small number of antennas at the wireless device. The large antenna array at the access node enables high-order transmission beamforming in the downlink.
The procedure of sequentially transmitting the beam in all necessary directions is referred to as a beam sweep or beam scan. A beam sweep may involve a variable number of beams depending on the situation. Often, quite many beams may be required, especially when the candidate beams originate from multiple candidate access nodes.
The beam sweep may serve other purposes than just time and frequency synchronization; in particular, the sweep may also serve the purpose of determining the best beam direction for data transmission to the new wireless device. In such cases, the beam may as mentioned above contain some information (e.g. a reference symbol sequence) that uniquely identifies the beam, so that the wireless device can report to the access node, which beam that was best received. Such a reference signal is in 5G sometimes referred to as a Mobility Reference Signal, MRS or Beam Reference Signal, BRS.
In beamforming-based communication networks, it could also be beneficial to avoid always-on signaling. In fact, in 3GPP, it has been agreed that the transmission of so-called always-on signaling should be minimized.
Accordingly, there is still a general demand for solutions as to how to manage reference signals in an efficient manner.